A sigil is a type of upgrade component used in weapons. Like all upgrade components, attaching a sigil to a weapon of masterwork, rare, or exotic rarity will make that weapon Soulbound; fine or basic weapons will become Account Bound. You can replace a sigil with a new one or recover it with a Black Lion Salvage Kit.

Do sigils stack?

Yes. Any Sigil can stack with any other sigil, in your case, Malice with Venom. The only rule is you cannot have the same named sigil on both of your active Sigil slots, EG, Malice/Malice is a no-go, as would Venom/Venom, etc.

What do you do with runes gw2?

Runes are upgrade components for armor that provide stat bonuses and other effects. Like all upgrade components, attaching a rune to armor of masterwork, rare, or exotic rarity will make that item Soulbound; fine or basic gear will become Account Bound.